The present invention is generally directed to aqueous ink compositions. More specifically, the present invention is directed to colored, especially pigmented aqueous ink compositions particularly suitable for use in ink jet printing processes, and especially thermal ink jet processes. In embodiments, the present invention is directed to processes for the preparation of anionically charged latexes with properties that enable the breakdown, or reduction in size of formed aggregates of latex and colorant, especially pigment particles upon aggregation and coalescence to provide submicron to micron sized pigmented particles in the size range of, for example, from about 0.3 to about 2, or about 1.5 microns (volume average throughout) as measured by a disk centrifuge.
The present invention is specifically directed to the preparation of latexes by, for example, batch or semibatch emulsion polymerization processes, wherein the parameters, such as temperature, monomer composition, chain transfer agent (CTA) type and concentration, and type and concentration of surfactant, permit products with colloidal or surface properties that result in submicron pigmented particles when aggregated and coalesced. The process for the preparation of an anionically charged latex comprised of resin, or polymer particles suspended in water, involves the mixing and selection of a suitable latex, and a water soluble initiator, which initiator provides for the majority of the surface charge on the latex. The charge is not believed to be primarily directed by the oil soluble initiator which is less ionic. The latex is combined with a dispersion of colorant, especially pigment, in an aqueous mixture containing an aqueous surfactant, such as a cationic surfactant, which surfactant is present in suitable amounts, such as for example, from about 0.5 weight percent throughout unless otherwise indicated, to about 10 percent and shearing this mixture in the presence of the latex mixture comprised of suspended submicron resin particles of from, for example, about 0.01 to about 0.8 micron in volume average diameter in an aqueous solution containing anionic surfactant, which surfactant is selected, for example, in an amount of from about 0.5 percent to about 10 percent, and nonionic surfactants selected in amounts of from about 0.02 percent to about 10 weight percent. The resultant mixture comprised of a flocculation of polymer, or resin and colorant, especially pigment particles, is then vigorously mixed with a high speed shearing device, followed by heating at, for example, from about 5 to about 40.degree. C. below the resin, or polymer Tg resulting in the formation of electrostatically bound aggregates of from about 0.5 to about 5 microns in volume average diameter, and which particles are comprised of colorant, such as pigment and polymer, followed by the optional, but preferable addition of a surfactant or stabilizer, and thereafter, heating the formed aggregates above about the polymer Tg, wherein the formed aggregates breakdown, or are separated and reduced to particles of resin and colorant suitable for ink jet inks. Breakdown refers to a change, especially reduction in size, or separation of the formed aggregates into about submicron to about micron sized particles containing colorant, wherein the size thereof is, for example, in the range of from about 0.3 to about 2.5 microns and preferably in the range of from about 0.3 to about 1.5 microns.
The present invention is further directed to processes wherein the amount of anionic surfactant, the type of anionic surfactant, the concentration of the water soluble initiator, the concentration of the chain transfer agent, such as carbon tetrabromide, and the amount of acrylic acid or methacrylic acid results in latexes with certain properties which when aggregated and coalesced result in a breakdown of the aggregates to submicron colorant, such as pigmented particles suitable for ink jet processes.
The latex can be prepared by batch or semibatch polymerization processes, wherein in the batch polymerization process the addition of (1) an organic phase comprised of monomers, such as styrene, butyl acrylates, methacrylates, and the like, an oil soluble CTA (charge transfer agent), such as dodecanethiol, and the partial water soluble component, such as carbon tetrabromide and the acrylic or methacrylic acid, to (2) an aqueous phase containing an anionic surfactant, such as sodium dodecylbenzene sulfonate (SDBS), and a nonionic surfactant, such as nanophenol ethoxylates, for example ANTAROX 897.TM., and emulsified for an effective period of time, for example from about 5 to about 20, and preferably about 10 minutes, prior to the addition of the initiator, such as potassium or ammonium persulfate and mixed, followed by polymerizing the monomers at a suitable temperature, for example, by heating at from about 60.degree. C. to about 80.degree. C. for a period of, for example, from about 6 to about 8 hours. The semibatch polymerization can provide greater process latitude for synthesizing custom latexes suitable for ink jet applications, primarily since, for example, a fraction of the organic and aqueous phases are added initially and the remainder is added over a period of time during the polymerization. Therefore, a greater control on the monomer composition, the distribution of the acrylic acid, the charge of the latex particles, and the like can be readily obtained since the feed composition to the reactor composition can be custom tailored to provide suitable ink requirements. Furthermore, the type of anionic surfactant at relatively the same concentration, for example from about 0.5 weight percent to about 10 percent, or parts can also result in the formation of submicron pigmented particles suitable for ink jet applications during coalescence by the known emulsion/aggregation processes, reference for example the patents recited herein.
In embodiments, the present invention also resides in processes for the preparation of submicron colorant, such as pigmented particles, wherein the latex possesses a zeta potential of from about -85 to about -120 millivolts (mV) and contains submicron resin particles, anionic, such as NEOGEN R.TM., and nonionic surfactant, such as ANTAROX 897.TM., and is blended together with a colorant, especially pigment dispersion containing submicron pigment particles and cationic surfactant, such as SANIZOL B.TM., at speeds of from about 500 to about 5,000 rpm, where the zeta potential of the blend formed is in the range of from about -45 to about -55 mV, followed by heating the mixture to a suitable temperature of, for example, from about 5 to about 20 degrees below the resin Tg to form aggregates of pigment and resin particles having a size of from about 0.5 to about 5 microns in volume average diameter. Anionic stabilizer is then added to the mixture to increase the zeta potential of the aggregates to from about -75 to about -90 mV, and the temperature of the mixture is increased to from about 5 to about 50 degrees above the resin Tg, resulting in a breakdown of the aggregates into colorant, such as pigmented particles suitable for thermal ink jet methods.
Embodiments of the present invention also include the elimination of the anionic surfactant which is added to the aggregates and prior to coalescence by modifying the surface or colloidal properties thereof and wherein such latexes when aggregated result in a zeta potential which is equal to that of the mixture that contained the additional anionic stabilizer, which when coalesced also results in an aggregate reduction, or breakdown; a process for the preparation of latexes which have surface colloidal properties suitable for the formation of submicron pigmented particles wherein the desired surface colloidal properties are obtained by the manipulation of processing conditions, such as
(i) the sequence of addition of the monomers;
(ii) concentration of the components utilized;
(iii) the concentration of the carboxylic acid utilized, which is, for example, from about 3 to about 20 weight percent of the monomers and preferably in the range of about 5 to about 15 percent;
(iv) the type, the concentration and the purity of the anionic surfactant, such as sodium dodecyl benzene sulfonate, which can be either linear or branched; and/or
(v) the ratio of the carboxylic acid, such as acrylic acid or methacrylic acid, to the partially soluble water chain transfer agent, such as carbon tetrabromide, and which ratio is, for example, greater than about 1:0.5 by weight percent of the monomers; and wherein the monomers are polymerized in a nonionic/anionic surfactant solution utilizing a water soluble initiator, such as an inorganic persulfate such as ammonium persulfate, and an optional organic soluble initiator, such as an azobis compound, and wherein there results a resin with a weight average molecular weight of from about 10,000 to about 60,000 and preferably in the range of from about 20,000 to about 40,000, and a Tg of between about 20.degree. C. and about 60.degree. C., and preferably in the range of from about 25.degree. C. to about 50.degree. C.; whereupon such latexes when aggregated with submicron colorant, such as pigment particles, result in particle sizes of about 3 to about 5 microns; which when stabilized with an anionic surfactant to primarily retain the particle size of the aggregates upon further heating rather results in a breakdown of the preformed aggregates into colorant, such as pigmented particles, with a size diameter of from about 0.3 to about 2 microns, and which particles are, suitable for ink jet inks.
In another embodiment thereof, the present invention is directed to an in situ process comprised of first dispersing a colorant, especially pigment, such as SUNSPERSE BLUE.TM., in an aqueous mixture containing a cationic surfactant, such as dodecyl benzyl, dimethyl ammonium chloride (SANIZOL B-50.TM.), utilizing a high shearing device, such as a Brinkmann Polytron, microfluidizer or sonicator, thereafter shearing this mixture with a latex prepared as illustrated herein and comprised of suspended resin particles, such as poly(styrene butadiene acrylic acid), poly(styrene butylacrylate acrylic acid) or poly(styrene isoprene acrylic acid), and which particles are, for example, of a size ranging from about 0.01 to about 0.5 micron in volume average diameter as measured by the Brookhaven nanosizer or a Disk Centrifuge in an aqueous surfactant mixture containing an anionic surfactant, such as sodium dodecylbenzene sulfonate (for example NEOGEN R.TM. or NEOGEN SC.TM.), and a nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy)ethanol (for example IGEPAL 897.TM. or ANTAROX 897.TM.), thereby resulting in a flocculation, or heterocoagulation of the resin particles with the pigment particles; and which, on further stirring for from about 1 to about 3 hours while heating, for example, from about 35 to about 45C, results in the formation of statically bound aggregates ranging in size of from about 0.2 micron to about 1.5 microns in average diameter size as measured by the Disk Centrifuge, where the size of the aggregated particles and their distribution can be controlled by the temperature of heating, for example from about 5.degree. C. to about 25.degree. C. below the resin Tg. Thereafter, there is added a colloidal stabilizer, such as additional anionic surfactant like sodium dodecylbenzene sulfonate (for example NEOGEN R.TM. or NEOGEN SC.TM.), in an amount of, for example, from about 0.2 to about 2 percent by weight of the aggregate suspension to prevent, or minimize further growth of the aggregates and heating the aggregate suspension from about 5.degree. C. to about 50.degree. C. above the resin Tg to provide for particle fusion or coalescence of the polymer and pigment particles yielding a suspension of pigmented latex composite particles in which various particle size diameters can be obtained, such as from about 0.1 to about 3, from about 0.2 to 1.5 microns, or submicron, that is about 1 micron, or less in volume average particle diameter. The aforementioned particle dispersions are especially useful for ink jet printing of colored images with excellent line and solid resolution.
While not being desired to be limited by theory, it is believed that the flocculation or heterocoagulation is caused by the partial neutralization of the colorant, such as pigment mixture containing the pigment and cationic surfactant absorbed on the pigment surface with the resin mixture containing the resin particles and anionic surfactant absorbed on the resin particles.
In another embodiment thereof, the present invention is directed to an in situ process comprised of first dispersing a pigment, such as SUNSPERSE BLUE.TM., in an aqueous mixture containing a cationic surfactant, such as dodecyl benzyl, dimethyl ammonium chloride (SANIZOL B-50.TM.), utilizing a high shearing device, such as a Brinkmann Polytron, microfluidizer or sonicator, thereafter shearing this mixture with a latex prepared as illustrated herein and comprised of suspended resin particles, such as poly(styrene butadiene acrylic acid), poly(styrene butylacrylate acrylic acid) or poly(styrene isoprene acrylic acid), and which particles are, for example, of a size ranging from about 0.01 to about 0.5 micron in volume average diameter as measured by the Brookhaven nanosizer or a Disk Centrifuge in an aqueous surfactant mixture containing an anionic surfactant, such as sodium dodecylbenzene sulfonate (for example NEOGEN R.TM. or NEOGEN SC.TM.), and nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy)ethanol (for example IGEPAL 897.TM. or ANTAROX 897.TM.), thereby resulting in a flocculation, or heterocoagulation of the resin particles with the pigment particles; and which, on further stirring for about 1 to about 3 hours while heating, for example, from about 35.degree. C. to about 45.degree. C., results in the formation of statically bound aggregates ranging in size of from about 0.2 micron to about 1.5 microns in volume average diameter size as measured by the Disk Centrifuge, where the size of the aggregated particles and their distribution can be controlled by the temperature of heating, for example, from about 5.degree. C. to about 25.degree. C. below the resin Tg, and thereafter, adding a colloidal stabilizer, such as additional anionic surfactant such as sodium dodecylbenzene sulfonate (for example NEOGEN R.TM. or NEOGEN SC.TM.) in an amount of from about 0.2 to about 2 percent by weight of the aggregate suspension to prevent further growth of the aggregates and heating the aggregate suspension at from about 5.degree. C. to about 50.degree. C. above the resin Tg to provide for particle fusion or coalescence of the polymer and pigment particles yielding a suspension of pigmented latex composite particles in which various particle size diameters can be obtained, such as from 0.1 to about 3, and preferably from about 0.2 to 1.5 microns in volume average particle diameter. The aforementioned particle dispersions are especially useful for ink jet printing of colored images with excellent line and solid resolution.